This invention relates to an illumination system and, more particularly, to an extra high output (EHO) mercury fluorescent lamp system utilizing multiple internal amalgam patches and external temperature control mechanisms to control the amalgam "cold spot temperature" and hence the internal mercury pressure within the low pressure (fluorescent) lamp envelope. The combination of the lamp structure and external control mechanism stabilizes the illumination output during both continuous and transient operation significantly reducing problems associated with mercury migration and depletion problems during startup.
Low pressure, mercury vapor fluorescent lamps are used in a variety of lighting applications. Of particular interest, for purposes of the present invention, is the widespread use of fluorescent lamps to illuminate documents being copied in a reprographic device.
In a conventional mercury fluorescent lamp, an electrical discharge, arc current, is generated through a mixture of low pressure mercury vapor and a fill gas typically argon, neon, krypton, xenon or mixtures thereof. The visible illumination output from the lamp depends, among other variables, on the mercury vapor pressure within the lamp tube. The mercury vapor pressure is generally controlled by maintaining a cooled area "cold spot" somewhere on the wall of the lamp envelope. It is known in the prior art that the optimum mercury pressure for maximum visible light output of a fluorescent lamp is approximately 7 mtorr (independent of current). This corresponds to a mercury cold spot temperature of approximately 35.degree. C. At this pressure and temperature, the light output from the lamp increases monotonically with the arc current. At cold spot temperatures higher or lower than the optimum, light output falls off. It is therefore desirable to maintain the cold spot temperature, and therefore the mercury pressure, at optimum at any lamp current and ambient operating temperature. Prior art techniques for accomplishing this function typically require a temperature-sensitive device such as a thermocouple, thermistor or thermostat to monitor the temperature of the cold spot. A feedback circuit providing closed loop control of a temperature-regulating device is used to maintain the optimum temperature.
While this technique has been shown to be successful at low to moderate lamp power loadings, higher lamp temperature generated at elevated power loadings cause severe illumination stability problems. For certain document reproduction applications, it is desirable to operate the fluorescent illumination source at extremely high power loadings. In the prior art techniques mentioned above, the power loadings for a high output (HO) T8 fluorescent lamp is usually less than 3.25 watts/linear inch, whereas in high power T8 EHO applications, power density can be up to 10 watts/linear inch of lamp lighted length. At this increased loading, the lamp wall temperature is greatly increased, requiring the use of active cooling devices such as fans, solid state (Peltier) coolers and the like. Additionally, the lamp is very sensitive to its axial thermal temperature profile. Changes in the axial temperature profile due to transient operation or environment can cause wide variation in light output along the length of the lamp, drastically affecting the lamp illumination stability.
In order to achieve better thermal control of a fluorescent lamp at high power loadings, it is known to incorporate an amalgam-forming material such as an indium patch, within the lamp envelope. The indium forms an amalgam with the mercury, thus chemically containing the mercury within the amalgam. The temperature at which mercury is released from the amalgam is significantly higher than the optimum lamp wall temperature of the conventional non-amalgam lamp (100.degree. C. versus 35.degree. C.). (The optimum temperature is somewhat adjustable by the amalgam material composition.) Thus, use of the amalgam fluorescent lamp significantly reduces the cooling requirements by providing an optimum cold spot temperature much closer to the lamp wall temperature at the high power loadings.
Representative of prior art publications using amalgams in the interior of fluorescent lamp envelope are:
U.S. Pat. Nos. 4,499,400 and 4,437,041 disclose incorporating a lead-tin-bismuth alloy within a solenoid electric field lamp to control the mercury vapor pressure. The amalgam is wetted onto an internal helical coil assembly (in the '400 patent) or as a patch on a conductive strip within the lamp. In U.S. Pat. No. 4,581,557, two patches are formed within an arc discharge tube which is contained within a high-density auto light transmissive envelope. The amalgam is used to control fluctuation in power supplied to the lamp. U.S. Pat. No. 3,860,852 uses a plurality of amalgams formed on metal strips attached to each of the electrode stems for the purpose of stabilizing temperature response. U.S. Pat. No. 4,827,313 discloses an amalgam fluorescent lamp with a single patch formed on the entire surface of the lamp and feed-back control circuitry to control amalgam temperature by varying power to an associated resistor heater sleeve.
One problem not addressed by the prior art is illumination instability caused by mercury migration within the lamp envelope during both operating and non-operating conditions. With a non-amalgam lamp, mercury is collected at the cold spot during operation. During off periods, when no power is applied to the lamp, the mercury gradually and eventually redeposits itself over the entire lamp (assuming an isothermal condition within the lamp). This uncontrolled deposition is sometimes called mercury migration and can cause severe and sometimes long term axial illumination degradation until all the mercury within the lamp is again controlled primarily by the cold spot. In an amalgam type lamp, the amalgam is such an effective collector that the mercury tends to remain collected in the amalgam even during extended off periods. While this prevents an "excess mercury condition" it can lead to "mercury starvation" conditions at the lamp ends causing reduced illumination output in these regions until the mercury is again redistributed by thermal, convective and electrical forces within the lamp.
According to a first aspect of the present invention, a multiple amalgam patch lamp is designed to provide several mercury storage sites along the linear lamp axis, each site contributing to the sustaining of proper mercury pressure in it's localized region. Also included is a control system to monitor and adjust the temperature of said amalgam sites. Because of the distributed nature of these mercury sources, mercury is rapidly available along the entire lamp length even after long power-off periods. According to another aspect of the invention, the physical positioning and geometry of the patches is designed to achieve an optimum axial illumination output of the lamp by individually controlling the temperature of the individual amalgam sites. More precisely, the invention relates to a monitoring and control system, for an amalgam fluorescent lamp, said lamp having a first and second amalgam patch formed on the interior surface of the lamp envelope and at opposite ends, and at least a third amalgam patch formed at a generally central location on the interior surface of said lamp envelope, and means for providing independent monitoring and controlling of the temperature at each of said amalgam patches so as to provide overall control of the operating temperature of the lamp.